1. Field of the Invention
The present invention relates to a battery device for detecting a voltage and an abnormality of a secondary battery.
2. Description of the Related Art
FIG. 3 is a circuit diagram illustrating a related-art battery device. The related-art battery device includes a secondary battery 11, an N-channel discharge control field effect transistor 12, an N-channel charge control field effect transistor 13, a battery state monitoring circuit 14, resistors 22 and 31, a capacitor 32, and external terminals 20 and 21. The battery state monitoring circuit 14 includes a control circuit 15, a discharge overcurrent detection circuit 16, a charge overcurrent detection circuit 116, an overcurrent detection terminal 19, a charge control signal output terminal 41, a discharge control signal output terminal 42, a positive power supply terminal 44, and a negative power supply terminal 43. The discharge overcurrent detection circuit 16 includes a comparator circuit 18 and a reference voltage circuit 17. The charge overcurrent detection circuit 116 includes a comparator circuit 118 and a reference voltage circuit 117.
The control circuit 15 includes resistors 504, 505, 506, 507, and 518, reference voltage circuits 509 and 516, comparator circuits 501, 508, and 514, an oscillator circuit 502, a counter circuit 503, logic circuits 510 and 520, a level shift circuit 511, a delay circuit 512, and NMOS transistors 517 and 519.
Next, an operation of the related-art battery device is described. When a load is connected between the external terminals 20 and 21, and a discharge current flows therethrough, a voltage of the external terminal 21 increases. The discharge current generates voltages between a drain terminal and a source terminal of the N-channel discharge control field effect transistor 12 and between a drain terminal and a source terminal of the N-channel charge control field effect transistor 13, and the total voltage is input to the discharge overcurrent detection circuit 16. When the discharge current is represented by I1, an ON-resistance value of the N-channel discharge control field effect transistor 12 is represented by R12, and an ON-resistance value of the N-channel charge control field effect transistor 13 is represented by R13, a voltage generated between the negative power supply terminal 43 and the external terminal 21 is expressed by I1×(R12+R13). When I1×(R12+R13) exceeds an output of the reference voltage circuit 17, the comparator circuit 18 inverts its output and outputs a discharge overcurrent detection signal to the control circuit 15.
A voltage of the overcurrent detection terminal 19 is equal to a voltage of the external terminal 21, and hence, when the discharge overcurrent flows to increase the voltage of the external terminal 21, the voltage I1×(R12+R13) of the overcurrent detection terminal 19 increases to be equal to or higher than an output voltage of the reference voltage circuit 516 or an output voltage of the reference voltage circuit 17. Then, an output of the comparator circuit 514, or both outputs of the comparator circuits 514 and 18 are inverted to detect a discharge overcurrent or a short circuit, thereby outputting a signal for turning off the N-channel discharge control field effect transistor 12 to the discharge control signal output terminal 42. In this manner, the discharge overcurrent or the short circuit is detected to protect the battery device from the discharge overcurrent. The output voltage of the reference voltage circuit 17 corresponds to a discharge overcurrent detection voltage, and the voltage of the reference voltage circuit 516 corresponds to a short-circuit detection voltage (see, for example, Japanese Patent Application Laid-open No. 2004-104956).
In the related art, however, when the discharge overcurrent is detected, a battery voltage decreases due to an internal resistance of the battery, and hence there is a problem in that the battery voltage may fall below a minimum operating voltage of the discharge overcurrent detection circuit 16 and the comparator circuit 18 may not operate normally to reduce the safety of the battery device. Further, when the current flows between the external terminals 20 and 21, a gate-source voltage of the N-channel charge control field effect transistor 13 fluctuates to fluctuate the ON-resistance, and hence there is another problem in that the accuracy of the overcurrent detection may be reduced.